We study the Arnold diffusion in a priori unstable near-integrable systems in a neighbourhood of a resonance of low order. We consider a nonautonomous near-integrable Hamiltonian system with n + 1/2 degrees of freedom, n ≥ 2. Let the Hamilton function H of depend on the parameter ε, for ε = 0 the system is integrable and has a homoclinic asymptotic manifold Γ. Our main result is that for small generic perturbation in an ε-neighborhood of Γ there exist trajectories the projections of which on the space of actions cross the resonance. By "generic perturbations" we mean an open dense set in the space of C r -smooth functions d dε ε=0 H, r = r 0 , r 0 + 1, . . . , ∞, ω. Combination of this result with results of [44] answers the main questions on the Arnold diffusion in a priori unstable case: the diffusion takes place for generic perturbation, diffusion trajectories can go along any smooth curve in the action space with average velocity of order ε/| log ε|.